1. Field of the Invention
The present invention relates to a method for forming a gate stack which minimizes or eliminates damage to the gate dielectric layer and/or the silicon substrate during gate stack formation. More particularly, the present invention relates to reducing temperature during the fabrication of the gate stack to eliminate the formation of silicon clusters within the metallic silicide film of the gate stack. The present invention also includes methods for dispersing silicon clusters prior to the gate etch step.
2. State of the Art
The operating speed of semiconductor devices in very large scale integration ("VSLI") and ultra large scale integration ("USLI") depends primarily on the resistivity of the conductive material (hereinafter "trace material") used to transmit signals from one circuit component to another circuit component. Additionally, in order to increase the circuit component density and/or reduce the complexity of the metal connections between the circuit components, a highly conductive trace material layer is required on the gate stack. Thus, the trace material must be a low-resistivity material.
Metallic silicides have recently become popular for use as low-resistivity trace material. Tungsten suicide ("WSi.sub.x ") has become a leading low-resistivity trace material. Various etching chemistries have been developed to pattern the WSi.sub.x to form such conductors as the digitlines or wordlines used in memory devices (see commonly-owned U.S. Pat. 5,492,597, hereby incorporated herein by reference). Other metallic silicides used in gate stacks include cobalt silicide ("CoSi.sub.x "), molybdenum silicide ("MoSi.sub.x "), and titanium silicide ("TiSi.sub.x "). These metallic silicides have lower resistivity and are easier to fabricate than other conductors used for this purpose. However, metallic silicides are prone to oxidization. Furthermore, the metal components of the metallic silicides react chemically when they contact other elements. These properties present several problems, including degradation of the semiconductor element and peeling of the metallic silicide film. To compensate for these problems, a polysilicon layer is usually disposed between a gate dielectric layer and the metallic silicide film, and a dielectric cap layer is usually disposed above the metallic silicide film to isolate the metallic silicide.
FIGS. 14-19 illustrate, in cross section, a conventional method of forming a gate stack having a metallic silicide film layer. FIG. 14 illustrates a gate dielectric layer 204 such as silicon dioxide (SiO.sub.2) grown (by oxidation) or deposited (by any known industry standard technique, such chemical vapor deposition or the like) on a silicon substrate 202. A polysilicon layer 206 is formed on top of the gate dielectric layer 204, as shown in FIG. 15. The polysilicon layer 206 is then subjected to an ion implantation with gate impurities (not shown). As shown in FIG. 16, a metallic silicide film 208 is deposited on the polysilicon layer 206. The structure is then subjected to a heat treatment for about 30 minutes at a temperature between about 850.degree. and 950.degree. C. for activation of the impurities in the polysilicon layer 206 and to anneal the metallic silicide film 208. The heat treatment temperature level is dictated by the temperature required to anneal the metallic silicide film 208. The annealing of the metallic silicide film 208 is used to reduce its resistivity.
As shown in FIG. 17, a silicon dioxide cap 210 is then deposited on the metallic silicide film 208 at temperatures over 600.degree. C. by chemical vapor deposition ("CVD"), low pressure chemical vapor deposition ("LPCVD"), or the like. A resist 212 is then formed and patterned on the silicon dioxide cap 210, as illustrated in FIG. 18. The layered structure is then etched and the resist 212 is stripped to form a gate stack 214, as illustrated in FIG. 19. However, this etching results in pitting on the gate dielectric layer 204. This pitting is illustrated in FIG. 20 wherein a plurality of pits 216 is distributed on the gate dielectric layer 204 between the gate stacks 214.
This pitting is also illustrated in FIG. 19. A pit in the dielectric layer 204 may be shallow, such as shallow pit 218. However, a deep pit, such as deep pit 220, can extend through the gate dielectric layer 204 and into the silicon substrate 202. The pitting into the silicon substrate 202 will cause junction leakage, refresh problems, and potential destruction of the component. At present, most gate dielectric layers are about 80 .ANG. thick. However, as semiconductor devices continue to be miniaturized, these gate dielectric layers will become thinner. As the gate dielectric layers become thinner, it is more likely that pitting will penetrate through the gate dielectric layer to contact the silicon substrate and cause the aforementioned problems.
Therefore, it would be advantageous to develop a technique which minimizes or eliminates pitting on the gate dielectric layer caused by gate stack etching, while using state-of-the-art semiconductor device fabrication techniques employing known equipment, process steps, and materials.